This invention relates to a coincidence circuit in a line synchronizing circuit arrangement for a picture display device for establishing a coincidence state of an incoming line synchronizing signal which originates from a synchronizing signal separator, and a locally generated line signal which originates from a line oscillator, said coincidence circuit comprising a coincidence detector which is only operative during part of the field period, and storage means which are coupled to an output of the coincidence detector.
A line synchronizing circuit arrangement of this type is known from European Patent Application 91,719, which corresponds to U.S. Pat. No. 4,535,358. In this patent the circuit arrangement is switchable between two states by means of the coincidence circuit, in which the pull-in time of the line oscillator has different durations. Other uses of a coincidence circuit are known: some are described in U.S. Pat. No. 4,520,393 in which a number of switching possibilities is mentioned, including, inter alia, switching of the synchronizing signal separator. As a rule a switch-over takes place when the coincidence circuit has established a coincidence state, i.e. that coincidence between the signals applied thereto has occurred one or more times, a coincidence being understood to be the at least partly simultaneous occurrence of an incoming line synchronizing pulse and a line oscillator pulse.
The coincidence circuit described in the European patent applicated cited above comprises a concidence detector which is inoperative during the field blanking interval in order to avoid an interference which might occur in this interval due to the field synchronizing pulses. In fact, since these pulses are broader tha the line synchronizing pulses, the quantity of information stored in the storage means, for example, a capacitor, could reach the decision level at which the said switch-over takes place. This erroneous situation could still prevail at the end of the field blanking interval so that an interference would be visible at the top of the displayed picture.
However, during the rest of the field period, the field trace time, another interference may occur, namely an interference produced upon reception of television signals with echo and/or adjacent channel signals. In the case of such a reception video signal remnants having a phase which is arbitrary with respect to the line synchronizing signal may reach the synchronizing signal separator. Interference signals may then be supplied by the separator to the coincidence detector, together with the useful synchronizing signal derived from the incoming video signal. It is found that the coincidence detector has been often incapable of making a usable distinction between such a disturbed state and the state in which such interference signals are absent, with the result that the line synchronizing circuit arrangement can switch over prematurely, which is a serious disturbance of the operation of this circuit arrangement.